Upper Canada included all of modern-day southern Ontario and all those areas of northern Ontario in the 'pays d'en haut' which had formed part of New France, essentially the watersheds of the Ottawa River, Lake Huron, and Lake Superior. It did not include any lands within the watershed of Hudson Bay.

Additional districts were created from the existing districts as the population grew until 1849, when local government mainly based on counties came into effect. At that time, there were 20 districts; legislation to create a new Kent District never completed. Up until 1841, the district officials were appointed by the Lieutenant-Governor, although usually with local input. A Court of Quarter Sessions was held four times a year in each district to oversee the administration of the district and deal with legal cases.

War of 1812 (1812–1815)

During the War of 1812 with the United States, Upper Canada was the chief target of the Americans, since it was weakly defended and populated largely by American immigrants. However, division in the United States over the war, a lacklustre American militia, the incompetence of American military commanders, and swift and decisive action by the British commander, Sir Isaac Brock, kept Upper Canada part of British North America.

Detroit was captured by the British on August 6, 1812. The Michigan Territory was held under British control until it was abandoned in 1813.

Dissidents and 1837 Rebellion

After 1800 there emerged a dissident faction that questioned the direction and handling of the colony by the Tories, including both colonial and imperial officials. The leaders were Robert Thorpe, Joseph Willcocks, Robert Gourlay, and especially, William Lyon Mackenzie. They challenged the establishment about taxes, land policy, the privileges of the Anglican Church and the Family Compact, appropriations, and freedom of the press. They claimed that all citizens and not just the enfranchised were entitled to a voice, but they did not form a political party; there were no parties. In 1831 the Tories expelled MacKenzie from the Assembly, and the conflict escalated. Their brief armed rebellion in 1837 failed. Officials blamed American influence, "In this country unfortunately the settlement of American citizens has been too much permitted and encouraged, and thus in the bosom of this community there exists a treacherous foe... in many parts of the Province the teachers are Americans.... These men are utterly ignorant of everything English and could not if they tried instruct their pupils in any of the duties which the connection of the Province with England casts upon them." [Rea] The oligarchic Family Compact was defended by Tories who explained, "The Radicals, Revolutionists or Destructives was composed of all the American settlers and speculators in land, some of the more simple and ignorant of the older class of farmers, and the rabble of adventurers who poured in every year from the United States or from Britain, to evade the laws of their respective countries." [Rea]
Lord Durham's support for "responsible government" undercut the Tories and gradually led the public to reject what it viewed as poor administration, unfair land and education policies, and inadequate attention to urgent transportation needs. Finally there emerged responsible government under Robert Baldwin and Louis LaFontaine by the late 1840s.

Rudimentary municipal administration began with the creation of districts, notably Western (including present day Brantford), Eastern, Gore (including present day Hamilton) and Home (including present day Toronto).

Organized settlement tracts were laid out with portions set aside for the clergy reserves, one exception was the Talbot Settlement on the north shore of Lake Erie which was set up in 1804.

These land tracts expanded in reach well beyond the St. Lawrence-Lake Ontario-Lake Erie shores after the war of 1812. In 1828, Britain appointed Upper Canada's first Chief Agent of Emigration, A.C. Buchanan. His title reflected the centrality of Britain's perspective on migration in the colonies at the time, especially since most new arrivals in Upper Canada were from the United Kingdom.

British regiment soldiers who were veterans of the war were offered free land, some remained despite the harsh winters. Unlike the period prior to the war, immigration was now directed at Europe and more specifically to the United Kingdom and Ireland, not from the US, which was the largest source of immigration before the war. Very cheap or even free land was offered with advertisements to entice immigrants to settle there, even those in financially meager circumstances. Passage could be obtained across the Atlantic on returning empty lumber ships for little fare. During the early 1830s, the population increased more than 10% of its total each year. In the 1820s many German speaking Mennonite immigrants came to the Grand River region of Upper Canada from Pennsylvania, they were joined as well by many German speaking Amish immigrants. This region was sometimes called "Little Pennsylvania", however this term is no longer used today. Many of their descendants continue to speak a form of German called Pennsylvania German.

It is estimated that thousands of escaped slaves entered Upper Canada from the United States, using the Underground Railroad.

Upper Canada ceased to be a political entity with the Act of Union (1840), when, by an act of the British Parliament, it was merged with Lower Canada to form the United Province of Canada. This was principally in response to the Upper and Lower Canada rebellions of 1837 and 1837-38, respectively. At Confederation in 1867, the Province of Canada was re-divided along the former boundary as the provinces of Ontario and Quebec.

The name 'Upper Canada' lives on in a few historical forms, most notably the Law Society of Upper Canada, Upper Canada Lumber, Upper Canada College, Upper Canada Mall (in Newmarket, Ontario), and the Upper Canada Brewing Company.
When the capital was first moved to Toronto from Newark (present-day Niagara-on-the-Lake) in 1796, the Parliament Buildings of Upper Canada were located at the corner of Parliament and Front Streets, in buildings that were burned by United States forces in the War of 1812, rebuilt, then burned again by accident. The site was eventually abandoned for another, to the west. In 2001, some remnants of the original Parliament building were found. Today, there is an ongoing fight by preservationists and historians who propose the government develop and interpret the historic site. Currently the government leases most of the site.

Canada West was a political entity and a geographic way of referring to the former Province of Upper Canada, following its merger into the United Province of Canada. Canada West was given 42 seats in the Legislative Assembly of Canada, exactly the same number as Canada East.

The population of Canada West had grown substantially during the period it existed, mostly due to unprecedented immigration and a high birth rate. In 1841 it was 455,000 but grew to approximately 1,500,000 people or more than 300% at the time of Confederation in 1867.

Censuses were conducted in 1851 and 1861 and the population in those years was 952,004 and 1,396,091 respectively.